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  1. JDK
  2. JDK-8215470

Bad EnclosingMethod attribute on classes declared in lambdas

    Details

    • Subcomponent:
    • Resolved In Build:
      b21
    • CPU:
      x86_64
    • OS:
      generic

      Description

      ADDITIONAL SYSTEM INFORMATION :
      Any OpenJDK version 1.8 onward, any platform. Tested and verified on 1.8.0_60-b27 and 18.9 (build 11+28) on Windows 7.

      A DESCRIPTION OF THE PROBLEM :
      If an anonymous or local inner class is declared inside of a lambda, the `EnclosingMethod` attribute on that class refers to the compiler-generated "lambda$" method rather than the method that enclosed the original declaration.

      This is a violation of JVMS §4.7.7 ("The EnclosingMethod Attribute"):

      > It is the responsibility of a Java compiler to ensure that the method identified via the `method_index` is indeed the closest **lexically enclosing** method of the class that contains this `EnclosingMethod` attribute.

      Emphasis mine. Currently, if the class containing an `EnclosingMethod` attribute was declared within a lambda expression, the attribute refers to a method that never existed in the lexical scope.

      A particularly vexing side effect of this occurs when the generic signature of a lambda-embedded inner type contains type variables declared by the lexically enclosing method. Those type variables will be out of scope from the perspective of the method identified by the `EnclosingMethod` attribute. Consequently, the `java.lang.reflect` APIs cannot resolve them.

      STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
      Run attached "source code for an executable test case". Note the output.

      EXPECTED VERSUS ACTUAL BEHAVIOR :
      EXPECTED -
      Output should be:

      [class LambdaBug$1] EnclosingClass=class LambdaBug; EnclosingMethod=static void LambdaBug.immediateScope()
      [class LambdaBug$1ExplicitToken] EnclosingClass=class LambdaBug; EnclosingMethod=static void LambdaBug.immediateScope()
      [class LambdaBug$2] EnclosingClass=class LambdaBug; EnclosingMethod=static void LambdaBug.lambdaScope()
      [class LambdaBug$2ExplicitToken] EnclosingClass=class LambdaBug; EnclosingMethod=static void LambdaBug.lambdaScope()

      ACTUAL -
      Output is actually:

      [class LambdaBug$1] EnclosingClass=class LambdaBug; EnclosingMethod=static void LambdaBug.immediateScope()
      [class LambdaBug$1ExplicitToken] EnclosingClass=class LambdaBug; EnclosingMethod=static void LambdaBug.immediateScope()
      [class LambdaBug$2] EnclosingClass=class LambdaBug; EnclosingMethod=private static void LambdaBug.lambda$lambdaScope$0()
      [class LambdaBug$2ExplicitToken] EnclosingClass=class LambdaBug; EnclosingMethod=private static void LambdaBug.lambda$lambdaScope$0()


      ---------- BEGIN SOURCE ----------
      class LambdaBug<T> {
          protected LambdaBug() {
              System.out.printf("[%s] EnclosingClass=%s; EnclosingMethod=%s%n",
                               getClass(),
                               getClass().getEnclosingClass(),
                               getClass().getEnclosingMethod());
          }

          static <X> void immediateScope() {
              class ExplicitToken extends LambdaBug<X> {}
              new LambdaBug<X>() {};
              new ExplicitToken();
          }

          static <X> void lambdaScope() {
              Runnable r = () -> {
                  class ExplicitToken extends LambdaBug<X> {}
                  new LambdaBug<X>() {};
                  new ExplicitToken();
              };
              r.run();
          }

          public static void main(final String[] args) {
              immediateScope();
              lambdaScope();
          }
      }
      ---------- END SOURCE ----------

      FREQUENCY : always


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              • Assignee:
                vromero Vicente Arturo Romero Zaldivar
                Reporter:
                webbuggrp Webbug Group
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                • Created:
                  Updated:
                  Resolved: